As a synthetic resin cap having a conventional one-piece structure, there is known a plastic cap having a substantially circular top plate and a skirt pending from a circumference of the top plate, which are integrally formed of plastic, the skirt being provided in its inner circumferential face with an engagement portion for fastening such as a thread with respect to a container, wherein in an inner corner part between the top plate and the skirt, an undeformable annular reception seat adapted to closely engage a part spanning from an upper edge to an outer circumference of a container mouth, and at least one projection portion for sealing projecting by a very small distance from a plane of the container mouth passing the plane of the reception seat, and situated on the reception seat with an annular groove interposed therebetween are provided, and when the cap is fastened to the container mouth, the projection portion for sealing is deformed by compression exclusively in a position where the container mouth and the reception seat engage each other, and hermetical sealing is achieved by the pressure accompanying the compression deformation of the projection portion for sealing (See Patent Document 1).    Patent Document 1: Japanese Examined Patent Publication No. 6-33098